Help Making Asteroids (3D) (Cellular Automata?)

I have a concept that I am working on, basically the part im coding right now is a spacecraft that can move through a chunk of empty space. Here the problem: I am a "Late Beginner", and I am trying to find out how to make asteroids. I was thinking Cellular Automata and something I heard of called Marching Squares, but I have no idea how to code that at all, and the examples I have seen of 3D Cellular Automata look like hills, not asteroids. One other thing I thought of was making some asteroids (30 or so) and spawning THOSE in randomly a set average distance apart.

How do I do this? Help would be much appreciated!! Please remember I'm still a relative Beginner.